This invention relates generally to radar systems and methods therefor; and more particularly to pulse doppler radar systems and methods therefor which provide unambiguous target range (R) and range rate (R).
The design and selection of the optimum transmission waveform for a radar system that detects and tracks targets in the presence of land and/or sea clutter is a generic problem which has faced radar designers for many years. Accuracy requirements for target range and range rate measurements normally dictate that the transmitted waveform be a periodic pulse train or a pulse of encoded signals. However, choosing such a pulse train presents problems in selecting the PRF and/or the RF (radio frequency) bandwidth so that the desired accuracy and clutter resolution are obtained but neither the range rate nor the range measurement is ambiguous. For example, if the PRF is selected high enough so that the range rate measurement is unambiguous then the unambiguous range interval will be too small for many applications. Alternately, if a low enough PRF is selected to provide an acceptable unambiguous range interval then the unambiguous range rate capability is too small for many applications. Also due to the relatively small frequency interval between clutter spectral lines in the low PRF transmission mode, clutter signals are likely to be in the same frequency range as, and thereby obscure target returns; i.e. the target signals are likely to be "doppler eclipsed" by the clutter signals. Further, in the high PRF mode there is a relatively short "listening" period between transmission pulses and target returns can be "range eclipsed" due to the receiver being "non-receptive" during the transmission mode.
One type of current radar system employs transmission waveforms which measure one of the parameters (range or range rate) precisely and the second parameter rather inaccurately; however, both parameters are determined unambiguously. The AWG-9 radar system manufactured by Hughes Aircraft Company is of the just described type. Another type of current radar system, i.e. the MK-23, Target Acquisition System (TAS) designed by Hughes Aircraft Company, measures both parameters precisely but one is ambiguous. The APG-63 radar system, also manufactured by Huges Aircraft Company, precisely but ambiguously measured both targets' characteristics (range and velocity) and ambiguities are then resolved by methodically shifting waveforms during the dwell period and/or using frame-to-frame computer association and correlation routines.